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(11) | EP 0 984 265 A2 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Method and apparatus for machining material |
| (57) A method for measuring the machinability of a material includes piercing a hole through
a material to be tested while simultaneously measuring a pierce time duration, T,
of the piercing step, and calculating a machinability number from the pierce time
duration. Also provided are methods for determining the machining speed of a material
and for machining a material which calculate a machining speed for a material based
upon the machinability number of the material. The methods used to measure the machinability
number and calculate a machining speed for a particular material can include any combination
of machining operations, including abrasive water jet cutting processes. Also provided
is an apparatus which detects the time duration a piercing force takes to create a
pierce-through condition through a material. The apparatus includes any of a pressure
sensor, an acoustic sensor, an optical sensor, a load cell, a mechanical switch, or
any combination thereof, to measure the pierce-time duration. |
[0001] This invention relates generally to machining methods and apparatus and more particularly to methods and apparatus for abrasive waterjet machining of engineering materials.
[0002] The machinability number is a property of a specific material which varies depending on the type of machinery operation, such as cutting, to be performed and is related to a number of materials properties. For abrasive waterjet machining of ductile materials, the machinability number is related primarily to flow stress of the material. For abrasive waterjet machining of brittle materials, the machinability is related to the fracture energy, grain size (or flow distribution for materials such as glass), modulus of elasticity and Poisson's ratio.
[0003] In a workshop environment in which a variety of workpiece materials are cut using an abrasive waterjet machining process, if a new type of material is to be cut for the first time, a number of empirical cutting tests and calculations must typically be performed to determine the machinability number of the material prior to performing the cutting operation.
[0004] J. Zeng et al. in the paper titled "The Machinability of Porous Materials by a High Pressure Abrasive Waterjet", Proceedings of the Wintger Annual Meeting of ASME, 1989, pp. 37-42, first introduced and incorporated the concept of a "Machinability Number" into parameter prediction methods for determining optimum Abrasive Waterjet (AWJ) machining criteria (e.g. abrasive particle flow rate and cutting speed) to be used in order to achieve a desired surface quality for different materials to be machined.
[0005] Some AWJ systems manufacturers and their customers have attempted addressing the problem of determining the machinability numbers of materials by providing databases of machinability number data from trial-and-error, empirical tests performed by experienced AWJ operators on specific customer materials. This, however, involves considerable effort which is both costly and time-consuming. Such databases are also cumbersome and tedious to use.
[0006] The development of computer software has facilitated the use of databases of empirically obtained machinability number data to calculate the optimum operating criteria for materials for which the machinability number has been previously obtained. In the case a new material for which the machinability number is unknown is to be cut or otherwise machined, the computer software may also be used to guide and assist an operator in conducting the empirical tests required to determine the machinability number from test pieces of the material.
[0007] The trial-and-error testing required to obtain machinability number data is wasteful, however, because it requires the use of numerous test pieces of material. Moreover, the testing is costly and time-consuming because it requires an AWJ machine operator to manually perform and evaluate the results of the testing, input the resultant empirical results, and program the machining parameters required for a subsequent machining operation to be performed.
[0008] According to one aspect of the present invention, there is provided a method for measuring the machinability of a material, comprising the steps of:
a) providing a material;
b) piercing said material;
c) simultaneously measuring a pierce time duration, T, of said piercing step; and
d) calculating a machinability number from said pierce time duration.
[0009] According to a second aspect of the present invention, there is provided a method for determining the machining speed of a material, comprising the steps of:
a) providing a material;
b) piercing said material;
c) simultaneously measuring a pierce time duration, T, of said piercing step;
d) calculating a machinability number from said pierce time duration; and
e) calculating a speed at which said material is to be machined from said machinability number.
[0010] According to a third aspect of the present invention, there is provided a method for machining a material, comprising the steps of:
a) providing a sample comprising a material to be machined;
b) piercing a hole through said sample;
c) simultaneously measuring a pierce time duration, T, of said piercing step;
d) calculating a machinability number for said material from said pierce time duration;
e) calculating a speed at which a workpiece comprising said material is to be machined from said machinability number; and
f) machining a workpiece comprising said material at said calculated speed.
[0011] According to a fourth aspect of the present invention, there is provided an apparatus for detecting the pierce time duration of a piercing force through a material which is being pierced, comprising a means for detecting a pierce-through condition through a material made by a piercing force and a timing means for detecting a pierce time duration of said piercing force to create said pierce-through condition.
[0012] For a better understanding of the invention and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which:-
Fig. 1 is a general diagram of the components of an abrasive waterjet system;
Fig. 2 is a representation of the pressure signal read by a pressure sensor during a cutting method performed according to one embodiment;
Fig. 3 shows a reproduction of the wave form actually generated by an acoustic sensor used in one embodiment;
Fig. 4 is a program flow chart for a software program resident in the programmable controlling unit of Fig. 1 for performing a cutting method; and
Figs. 5 and 6 are general diagrams showing alternative embodiments of pierce-through detection devices.
[0013] Fig. 1 shows a broad system diagram of an embodiment as applied to an abrasive waterjet (AWJ) system. Abrasive water jet (AWJ) processes employ abrasive materials entrained into a high-pressure waterjet to perform a variety of cutting and other machining operations on a variety of materials. The high-energy waterjet beam utilised combines a rapid erosion of a workpiece material by high speed solid particle impacts with rapid cooling provided by a waterjet. In AWJ cutting operations an abrasive waterjet pierces through the thickness of and is then moved along a material to be cut.
[0014] Briefly, shown in Fig. 1 is a nozzle assembly 10 comprising an orifice 12 and a focussing tube 14 which applies a mixture of high pressure water and abrasive to a moving workpiece 16. The nozzle assembly 10 is preferably supplied with abrasive from an optional vibration feeder 10 and high pressure water from a water source 22. Although shown using a vibration feeder, it will be appreciated that other types of feeding devices may be used for this purpose.
[0015] In performing AWJ processes, a number of parameters such as water pressure, abrasive particle size, abrasive flow rate, and the dimensions of the waterjet nozzle office are varied depending on the type of material to be cut. A controlling unit 30 is provided which controls the feed from vibration feeder 20 and the feed supply of high pressure water from water source 22. Prior to performing an AWJ cutting operation, controlling unit 30 is typically preset by a user with the AWJ system operating parameters including "do", "df", "p", "A", and "q". The selection of these operating parameters for performing cutting operations using an AWJ apparatus are described in detail below.
[0016] In operation, as high pressure water and abrasives are supplied to the nozzle, the workpiece 16 is moved back and forth by positioning equipment (not shown), which manoeuvres the workpiece for the desired cutting operation responsive to a control signal provided by the controlling unit 30. The controlling unit 30 receives input on the operating conditions of the AWJ system and calculates optimum cutting speeds "uc" for the material of workpiece 16 and accordingly controls the motion of the workpiece 16 to provide the desired speed. The controlling unit 30 is preferably a Computerised Numerical Controller (CC) and may include, e.g. a Model ACR 2000 motion controller which is available from Acroloop Motion Control Systems, Inc., Chanhassen, MN, U.S.A.
AWJ Cutting Operations and Operating Parameters
[0017] As reported by J. Zeng and J.P. Munoz in the article titled "Intelligent Automation of AWJ Cutting for Efficient Production", Proceedings of the 12th International Symposium on Jet Cutting Technology, BHRA, Rouen, France, 1994, pp. 401-408, the traverse cutting speed "uc" at which a waterjet cuts through a particular material during an abrasive waterjet cutting operation may be estimated according to the following equation:
where "do" is the bore diameter (inches - one inch = 2.54cm) of orifice 12; "Df" is the bore diameter (inches) of the focussing tube 14; "P" is the water pressure (ksi) to be provided by high-pressure water source 22; and "A" is the abrasive flow rate (lbs/min - one lb = 0.4536 kg) to be provided to nozzle assembly 10 by the vibration feeder 20. "C" is a constant (which is 180.33 when calculating cutting speed using dimensions for an inch-unit system) and "q" is the quality index which is a measure of the desired surface condition of the resultant cut surfaces, "h" is the workpiece thickness, and "Nm" is the machinability number for cutting the workpiece material.
[0018] Typical operating parameters for performing an AWJ cutting process using the apparatus described above are as follows:
[0020] Orifice Bore Diameter (do) default set to 0.014 in (0.3556mm), but varies depending on water pump capacity.
[0021] Focussing Tube Bore Diameter (df): use tube having inner bore diameter approximately equal to 3 . do" (i.e., 0.042 inches - 1.0668mm).
[0023] For a cutting operation, the value of "q" in Eqn. 1 above can be chosen between 1 and 5 depending on the desired quality level. As described by J. Zeng et al. at pp. 174-175 of the article entitled "Quantitative Evaluation of Machinability in Abrasive Waterjet Machining", PED-Vol. 58, Precision Machining: Technology and Machine Development and Improvement, ASME 1992, pp. 169-179, the various quality levels are generally defined for engineering materials as follows:
| Quality Level | Description |
| q=1 | Criteria for Separation cuts - preferably q>1.2 should be used |
| q=2 | Rough surface finish with striation marks at lower half surface |
| q=3 | Smooth/Rough transition criteria - Slight striation marks may appear |
| q=4 | Striation-free for most engineering materials |
| q=5 | Very smooth surface finish |
[0024] The thickness "h" is dictated by the size of the workpiece to be cut and is measured and inputted into controlling unit 30 either manually by a user or automatically using a thickness sensor as described in detail below.
[0025] The machinability number "Nmc" depends upon the type of the material upon which a cutting operation is to be performed. Because the machinability number is a property of a specific material, it must be determined prior to performing a cutting operation on a workpiece made of a type of material which has not been cut before. With the present method, the need for predetermining a machinability number for a material by multiple trial-and-error tests prior to performing a machining operation may be eliminated by the automatic machinability measuring and machining methods and the apparatus therefor provided herein.
[0026] The present inventor has ascertained that the machinability number "Nmc" for a cutting operation on a material is inversely related to the time "T" required by an AWJ waterjet to pierce through a material using a circular piercing motion having a thickness "h" according to the following empirical equation:
where "Ctc" is a constant (which is 54.7 when calculating cutting speed using dimensions for an inch-unit system) and "P" is the waterjet pressure (ksi) and "do" the orifice diameter (inches) of the water jet nozzle. Thus, by making an initial test hole through an unknown material and incorporating a means for detecting the moment the waterjet pierces through the material, the machinability number for cutting can be automatically calculated for a workpiece based upon the piercing time. Moreover, if the piercing operation is performed on the actual workpiece to be cut the test hole may be used as a starting hole for the cutting operation. In this manner, the need for any preliminary testing on scrap material may be eliminated by a "drop-and-cut" operation in which a workpiece of unknown material is simply placed on the AWJ apparatus and cut by an automated process.
Apparatus for Detecting Pierce-Through of a Workpiece
[0027] In order to detect the moment a waterjet pierces a workpiece, a number of means may be incorporated. According to one embodiment, as shown in Fig. 1 a nozzle assembly 10 includes a nozzle shield 15 surrounding the focussing tube 14. The nozzle shield 15 is connected to and in fluid communication with an air or other gas supply 40 via a conduit 41. A pressure sensor 42 is connected to the conduit 41 and located between the nozzle shield 15 and providing a pressure sensor signal 43 to a controlling unit 30.
Operation for Performing Automated AWJ Cutting
[0028] Operation of the AWJ apparatus shown in Fig. 1 will be described with respect to performing an AWJ cutting operation according to the present automated method. Turning to the flow diagram in Fig. 4, controlling unit 30 is activated in Step 100 by inputting the specific AWJ system operating parameters including "do", "Df", "P", "A" and "q" prior to beginning an AWJ cutting cycle.
A. Positioning Nozzle Assembly at a Predetermined Stand-Off Distance
[0029] The controlling unit 30, upon receiving a user instruction to begin a cutting sequence, begins a piercing cycle in Step 110 by generating a control signal 39 in Step 110 to the air supply 40 thereby initiating airflow into the nozzle shield 15 via the conduit 41. The pressure sensor 42 generates and provides to the controlling unit 30 a signal similar to that shown in Fig. 2 indicating the pressure condition inside the nozzle shield 15 as a function of time. The controlling unit 30 generates a control signal 11 instructing motion equipment (not shown) to a lower nozzle assembly in Step 120 to form an air gap 17 having a predetermined height.
[0030] For a cutting operation, the nozzle shield 15 is set to establish a stand-off distance (i.e., the distance between the focussing tube 14 and workpiece 16) which is about equal to the air gap 17 once the air gap 17 is established. This is accomplished by using the pressure sensor 42 as a proximity switch which monitors in Step 130 the pressure increase caused by the restriction created between the workpiece 16 and nozzle shield 15 as it moves toward the target surface. As shown in Fig. 2, the pressure inside the nozzle shield 15 increases to a predetermined pressure Pg which is programmed into the controlling unit 30 and corresponds to the pressure at which the desired air gap 17 is formed. At this point, when Step 130 detects that the nozzle assembly 10 is in position, the controlling unit 30 generates a control signal to stop the motion of the nozzle assembly 10 thereby setting the cutting position (i.e. stand-off distance) and the controlling unit 30 also records this position.
B. Determining Workpiece Thickness
[0031] As discussed above, the thickness "h" of the workpiece 16 may be automatically measured and inputted into the controlling unit 30. This is accomplished in Step 140 by comparing the height of nozzle assembly in the cutting position with a known reference position and calculating the thickness of the workpiece. Alternatively, the thickness "h" may be measured manually by the operator and inputted into the controlling unit 30 in Step 140.
C. Piercing the Workpiece and Calculating Machinability Number
[0032] In Step 150, the controlling unit 30 simultaneously generates control signals 21 and 19 to initiate, respectively, the supply of high pressure water from the water source 22 and abrasive from the vibration feeder 20 to establish an abrasive water jet in the water nozzle assembly 10. The controlling unit 30 also begins a timing sequence and sends a control signal 11 to move the nozzle assembly 10 at a constant rate (e.g. 50 inches per minute - 127cm/min) along a circle which, preferably, has a radius equal to the focussing tube diameter, "Df", until the workpiece is pierced. During the time the piercing operating is being performed, the air supply 40 continues to provide a steady flow of air to the nozzle shield 15 while the pressure sensor 42 monitors and provides a steady output signal 43 to the controlling unit 30 as represented by the horizontal signal between the "tg" and Tp" shown in Fig. 2. Upon penetration (i.e. "pierce-through"), of the waterjet through the workpiece 16, a vacuum is created within the nozzle shield 15 which, as shown in Fig. 2, causes a virtually instantaneous drop in the pressure detected by the pressure sensor 42 at "tp", which is the moment pierce-through occurs.
[0033] Upon detecting the decrease in pressure in Step 160 caused upon pierce-through, the controlling unit 30 stops the timing sequence in Step 170 and obtains and records the pierce-through time "T". In Step 180, the controlling unit 30 calculates the machinability number "Nmc" and in turn the cutting speed "Uc" according to Eqns. 2 and 1 set forth, respectively, above.
D. Cutting of the Workpiece and Stand-Off Distance Monitoring
[0034] The desired cutting operation is then initiated in Step 190 by the controlling unit 30 which either sends a control signal 11 to begin horizontal movement of the nozzle assembly 10, sends a control signal 26 to begin horizontal movement of the workpiece 16, or both, to laterally move the nozzle assembly 10 at the calculated cutting speed "Uc" relative to the workpiece 16. The cutting operation is monitored in Step 200 either visually or automatically (e.g. by a mechanical sensor switch) to detect when the cutting operation is complete. During the cutting operation, the air gap 17 between the nozzle assembly 10 and workpiece 16 is preferably monitored in Step 210 for any changes by monitoring the signal provided by the pressure sensor 42 for any variation in the signal after time "tc" which represents the time at which pierce-through is completed and cutting begins as shown in Fig. 2. Should any variation above or below a predetermined pressure range (represented as "ΔP" in Fig. 2), which range corresponds to an acceptable stand-off distance tolerance, an error signal is sent by the controlling unit 30 via connection 11 to implement compensation in Step 220 by the motion equipment to adjust the stand-off distance. Alternatively, the controlling unit 30 may be programmed to send an error signal via connections 19 and 21 to respectively stop the flow of abrasive and water to interrupt the AWJ operation being performed.
Pressure Sensors and Alternative Pierce-Through Detection Devices
[0035] With respect to devices which may be incorporated as pressure sensor 42, any sensor which can detect the decrease of pressure which occurs within nozzle shield 15 upon pierce through may be incorporated. A typical device includes, but is not limited to, a Model OKC-424 Air Proximity Sensor available from O'Keefe Controls Co., Monroe, CT, U.S.A.
[0036] Although described above with respect to the use of the pressure sensor 42 and air supply 40 for detecting pierce time, it will be readily understood that other sensors may be incorporated to detect the movement of pierce-through by a waterjet. Shown in Fig. 5 are alternative pierce-through detectors 50 and 55 which, respectively, may be attached to or used in the vicinity of a workpiece 16 upon which an AWJ operation is being performed. The pierce-through detectors 50 and 55 may be used individually or in combination to provide a signal 51 to the controlling unit 30 at the moment of pierce-through. Typical detectors which may be used in this fashion include, but are not limited to conventional sensors, which can either directly detect the presence of the waterjet upon pierce-through (e.g. by means of a optical sensor) or indirectly detect some characteristic change which occurs upon pierce through (e.g. by means of an acoustic sensor or a load cell).
[0037] Acoustic sensors useful in this regard are those which can detect the change in sound level which occurs upon pierce through and include an acoustic sensor such as the Model 2800 Integrating Sound Level Meter available from Quest Technologies, Inc., Oconomooc, WI, U.S.A. Shown in Fig. 3 is a reproduction of the acoustic readings obtained during an AWJ piercing operation using an acoustic sensor with the initiation of the waterjet operation and the moment of pierce-through clearly indicated by two separate and distinct peaks. Load cells also useful in this regard are those which can detect the decrease in the force exerted on the workpiece 16 by a waterjet which occurs upon pierce-through and may include a waterjet which occurs upon pierce-through and may include load cells such as those available from Sens-All, Inc., Southington, CT, U.S.A.
[0038] Alternatively, as shown in Fig. 6 a conventional mechanical switch 60 may be located directly under the workpiece 16 such that, upon penetration by the waterjet, the switch 60 is tripped thereby indicating to the controlling unit 30 by signal 61 the existence of a pierce-through condition.
[0039] As a result of the present apparatus and method, an AWJ system is provided which provides a number of advantages over known cutting and other machining processes. Among these advantages is the ability to determine using the present automated machining processes which determine the optimum machining parameters for an AWJ operation without the need for multiple trial-and-error testing or extensive user experience for machining new materials. Moreover, the ability to "place and cut" a workpiece by using an initial starting hole as a test hole to determine the optimum cutting parameters for an AWJ cutting operation, thereby provides for a continuous operation.
[0040] Moreover, automatic programming of the machining speeds during operation may be accomplished without the need for any user interference or interface while also increasing the accuracy of the optimum machining speeds so determined. Furthermore, various control and measurement functions may be automatically accomplished using the present apparatus and method including, proximity detection of the waterjet nozzle, measuring the thickness of a workpiece, and real time monitoring and correction of nozzle stand-off distance. Additionally, compensation for changes in process parameters (e.g. changes in water pressure, abrasive flow rate, abrasive type, nozzle diameter, etc.) may also be made automatically.
Alternative Embodiments to Additional Machining Technologies
[0041] Although the method of determining the machinability number of a material is described above with respect to piercing the material using an AWJ waterjet moved through a circular motion, it is expected that other patterns of piercing motions may be employed as long as Eqn. 2 is empirically correlated with the type of piercing motion pattern selected. Such piercing motion patterns may include, but are not limited to, a linear, back-and-forth, star, wiggle or other pattern. Other modifications may also include the application of the apparatus and method for determining the machinability numbers using other energy beam machining technologies in addition to AWJ cutting processes. Moreover, it is envisaged that the determination of machinability numbers of engineering material using one energy beam process may be applied to or otherwise correlated with calculating process parameters for use in other types of energy beam machining processes.
[0042] Such energy beam technologies include those which utilise a concentrated beam energy to effect material removal to cut or otherwise make, shape, prepare or finish (i.e. machine) a raw stock material into a finished material. By way of example, it is envisaged that the apparatus and method may be adapted for incorporation into other types of energy beam technologies, including pure waterjet, laser, plasma arc, flame cutting and electron beam technologies. Although each of these use different physical phenomena to remove material, they behave similarly in nature and methodology to a waterjet energy beam such that the present apparatus and method may be employed.
[0043] Furthermore, it is to be understood that the selection of other energy beam technologies to which the present method and apparatus may be applied is not limited to these specific examples. These energy beam technologies may be selected by analysing the following features relative to an AWJ process:
| Energy Delivery Relationships: | |
| AWJ | The higher the applied flow/pressure (hp/watts), the faster the material removal rate. |
| Laser | The higher the applied output power (hp/watts), the faster the material removal rate. |
| Plasma | The higher the applied flow/pressure (hp/watts), the faster the material removal rate. |
| Material Removal Rate Relationships | |
| AWJ | The thicker the material, the slower the cutting speed. |
| Laser | The thicker the material, the slower the cutting speed. |
| Plasma | The thicker the material, the slower the cutting speed. |
| Surface Finish Relationships: | |
| AWJ | The faster the cutting speed, the rougher the cut surface finish. |
| Laser | The faster the cutting speed, the rougher the cut surface finish. |
| Plasma | The faster the cutting speed, the rougher the cut surface finish. |
| Analytical Relationships: | |
| AWJ | Cutting and machining (e.g. turning) removal rates can be related to a machinability number. |
| Laser | Cutting and machining removal rates can be related to a machinability number. |
| Plasma | Cutting rates can be related to a machinability number. |
[0044] Thus, it is envisaged that the present method and apparatus may be used to determine the machinability number for other energy beam processes (e.g. later and plasma energy beam processes) which cause material responses similar to those described above for a waterjet process. Additionally, the energy beam processes including AWJ may be used to perform a variety of other AWJ and traditional operations such as piercing, drilling and turning operations.
1. A method for measuring the machinability of a material, comprising the steps of:
a) providing a material;
b) piercing said material;
c) simultaneously measuring a pierce time duration, T, of said piercing step; and
d) calculating a machinability number from said pierce time duration.
2. A method for determining the machining speed of a material, comprising the steps of:
a) providing a material;
b) piercing said material;
c) simultaneously measuring a pierce time duration, T, of said piercing step;
d) calculating a machinability number from said pierce time duration; and
e) calculating a speed at which said material is to be machined from said machinability number.
3. A method for machining a material, comprising the steps of:
a) providing a sample comprising a material to be machined;
b) piercing a hole through said sample;
c) simultaneously measuring a pierce time duration, T, of said piercing step;
d) calculating a machinability number for said material from said pierce time duration.
e) calculating a speed at which a workpiece comprising said material is to be machined from said machinability number; and
f) machining a workpiece comprising said material at said calculated speed.
4. A method according to claim 1, 2 or 3, wherein said piercing step is performed by
an abrasive waterjet process.
5. A method according to claim 1, 2 or 3, wherein said piercing step is performed using
a concentrated beam energy.
6. A method according to claim 3, wherein said step of machining is performed using a
concentrated beam energy.
7. A method according to claim 5 or 6, wherein said concentrated beam energy is a beam
energy selected from the group consisting of an abrasive waterjet, a laser, a plasma
arc, a flame and an electron beam.
8. A method according to claim 3, wherein said step of machining is a machining operation
selected from the group consisting of a piercing, a drilling, and a turning operation
or any combination thereof.
9. A method according to any one of the preceding claims, wherein said step of calculating
a machinability number is according to the following equation for obtaining the machinability
number, Nmc, for a cutting process:
10. A method according to claim 9, wherein said cutting process is an abrasive waterjet
cutting process.
11. A method according to claim 10 as appendent to claim 2 or 3, wherein said step of
calculating a speed at which said material is to be machined is according to the following
equation to obtain a cutting speed, uc, for an abrasive waterjet cutting process:
12. A method according to claim 3 or any one of claims 4 to 11 as appendant to claim 3,
wherein said workpiece which is machined in said machining step comprises said sample
which is pierced in said piercing step.
13. An apparatus for detecting the pierce time duration of a piercing force through a
material which is being pierced, comprising a means for detecting a pierce-through
condition through a material made by a piercing force and a timing means for detecting
a pierce time duration of said piercing force to create said pierce-through condition.
14. An apparatus according to claim 13, wherein said means for detecting comprises a shield
means (15) for surrounding a source of said piercing force, a means (40) for supplying
a gas to create a pressure within said shield means, and a sensing means (42) for
detecting a decrease in pressure caused within said shield means by said pierce-through
condition created by said piercing force.
15. An apparatus according to claim 14, wherein said sensing means (42) is disposed between
and in fluid communication with said means for supplying gas (40) and said shield
means (15).
16. An apparatus according to claim 13, 14 or 15, wherein said means for detecting a pierce-through
condition is selected from the group consisting of an acoustic sensor, an optical
sensor, a load cell, a mechanical switch or any combination thereof.